Porous silicon is found in a variety of applications where its benign nature, electronic and optical properties and entrapment of other materials is desirable.
Porous silicon can exist in a crystalline form (e.g. polycrystalline porous silicon) or non-crystalline form (i.e. amorphous porous silicon). Amorphous porous silicon tends to be more reactive than crystalline porous silicon. In addition, amorphous and crystalline porous silicon possess different optical properties. The presence or absence of a crystalline phase may be readily ascertained using x-ray diffraction.
There are numerous methods available for making porous silicon. For example, and as described in PCT/GB96/01863, bulk crystalline silicon can be rendered porous by partial electrochemical dissolution in hydrofluoric acid based solutions. This etching process generates a silicon structure that retains the crystallinity and the crystallographic orientation of the original bulk material. Hence, the porous silicon produced is a form of crystalline silicon.
Another conventional method for making porous silicon is the so-called stain-etching technique. This method involves the immersion of a silicon sample in a hydrofluoric acid solution containing a strong oxidising agent. No electrical contact is made with the silicon, and no potential is applied. The hydrofluoric acid etches the surface of the silicon to create pores.
It is also known to form crystalline porous silicon from porous silica. Typically, this involves the use of reducing agents at high temperatures. Such processes result in highly crystalline porous silicon.
There is a continued need for alternative and preferably improved methods for making porous silicon. It is an object of the present invention to provide a method for making mesoporous silicon, particularly mesoporous amorphous silicon, wherein preferably the method is more economically viable than existing methods. Further, the present invention is based partly on the surprising finding that mesoporous amorphous silicon may be made from mesoporous amorphous silica using a strongly exothermic reaction. Optionally, the silica is sourced from a cheap, sustainable and readily available source.